The present application relates to a hologram reproducing apparatus, a hologram reproducing method, and a phase modulating element.
In recent years, hologram memories have become a focus of attention as storage devices enabling high-density recording/reproducing. As one of techniques of recording/reproducing information on/from a hologram memory, a technique of recording information by removing zero-order light (DC component) and reproducing it by superimposing coherent light on diffracted light has been suggested (see Non-patent Document 1: Joby Joseph and David A. Waldman “Homogenized Fourier transform holographic data storage using phase spatial light modulators and methods for recovery of data from the phase image” APPLIED OPTICS/Vol. 45, No. 25/1 Pp6374-Pp6380 September 2006; and Patent Document 1: Japanese Unexamined Patent Application Publication No. 2006-209081).
In the technique described in Non-patent Document 1, information is recorded by using a phase modulator, not a spatial modulator to modulate light intensity. In a case of using such a phase modulator, if half of pixels have a phase of 0 and if the other half of pixels have a phase of π, for example, no zero-order light is generated because laser light beams of opposite phases cancel each other out. On the other hand, in a case of recording information without using a phase modulator, recording by zero-order light is repeated at the same position of a medium, and recording cannot be performed any more after M/# at that position has been used up. Such inconvenience can be avoided by using the phase modulator and M/# can be effectively used advantageously. During reproducing, if the intensity of added light is the same as that of diffracted light and if the phase thereof is 0, the intensity of light in the pixels having a phase of 0 is high in the diffracted light, while the intensity of light in the pixels having a phase of π is low due to cancelling, so that recorded information can be reproduced as light and dark patterns. The optical system disclosed in Non-patent document 1 uses a dual beam method.